I think you should start more concentrated then if the "main" forums start to get too busy, then spin off sub forum categories. But is think the whole site should only be like.

US Politics
- Current events
- election
- issues
World Politics
- Russia and Europe
- Asia
- Africa
- Asia
- Canada
- Central and South America
Political Science
Off Topic chat
Feedback
Marketplace (if you want the trouble)

And that should put some lively discussion in a couple of forums at the top and leave room for everyone worldwide. Just my 2 Cents